Part 1:

Question 1. (a) A small reheating furnace wall consists of 200 mm of firebrick. The inner surface of the wall is at a temperature of 320 °C and the outside temperature is 35 °C. Calculate the rate at which heat is transferred, by conduction, through unit area of the wall. The thermal conductivity of the firebrick used can be taken as 1.8 W m-1 K-'.
If the outside surface area of the furnace is 50 m2 estimate the heat losses through the furnace wall per hour.
(b) A furnace wall consists of three layers of material as shown below.

Question 2 A pipe carrying superheated steam at 300 °C has an outside diameter of 120 mm and is lagged with two layers of insulating material. The first layer (adjacent to the outer pipe wall) is 25 mm thick and has a thermal conductivity of 0.072 W m--1 K-1. The second layer (covering the first layer) is 20 mm thick, has a thermal conductivity of 0.051 W m-i K-1 and an outside temperature of 28 °C.
Estimate the rate of heat loss per metre length of pipe (assume the thermal resistance of the pipe wall is negligible).

Question 3 (a) The Grashof number and the Reynolds number appear in most correlations of experimental data for convective heat transfer. Explain, in a maximum of 150 words, the mechanisms of natural and forced convection with particular reference to the above non-dimensional groups.
(b) An appropriate correlation for heat transfer by natural convection from a horizontal pipe to the atmosphere is
(C) The outer surface of the insulation on a horizontal steam pipe has a radius of 50 mm and is at a temperature of 90°C. The atmospheric air surrounding the pipe is at a temperature of 14°C, and has the property values listed in part (c) above. Estimate the rate of heat loss by natural convection to the atmosphere by each metre length of pipe.

Question 4. Liquid ammonia is heated as it flows at a mean velocity of 2 m s-1 through a circular pipe. The pipe, which has an internal diameter of 75 mm, is at a uniform temperature of 27°C, and the ammonia at a section 1.2 m from the inlet to the pipe has a temperature of -23°C. Use the following information to estimate the local heat transfer flux at / = 1.2 m. Note, the properties of ammonia liquid have been taken at -23°C, except where stated.

Part 2:

Question 1. (a) The data in TABLE 1 below relates to a specific heat exchanger. A reliable colleague has looked up an effectiveness chart and says that the effectiveness in the given operating conditions is 0.82.

Determine:
(i) the two outlet temperatures (ii) the heat transfer rate.
(b) Another colleague, who is not altogether reliable, has analysed theheat exchanger, referred to in Question 2 (a), using the correctionfactor method and he claims that the correction factor is 0.595. Confirm whether he is correct or not.

Question 2. (a) Dry saturated steam at a temperature of 180ºC is to be produced in a fire tube boiler from the cooling of 50 000 kg h-1 of flue gases from a pressurised combustion process. The gases enter the tubes of the boiler at 1600ºC and leave at 200ºC. The feed water is externally preheated to 180ºC before entering the boiler.
The mean specific heat capacity of the flue gases is 1.15 kJ kg-1 K-1.
The latent heat of vaporisation of the water at 180ºC is 2015 kJ kg-1. Feed water temperature = 180ºC.
Determine the amount of steam produced per hour, if the total heat loss is 10% of the heat available for steam raising.
(b) The overall heat transfer coefficient based on the outside area of the tubes is given as 54 W m-2 K-1. Determine the area of heat transfer required to perform this duty.
(c) The tubes within the boiler are to be 25 mm inside diameter with a wall thickness of 3 mm. The average flue gas velocity through the tubes to maintain the overall heat transfer coefficient value and to minimise pressure losses is to be more than 22 m s-1 and less than 28
m s-1.
Assuming that the average density of the flue gases is 1.108 kg m-3, calculate:
(i) the minimum and maximum number of tubes in each pass
(ii) the overall length of tubes at each of these numbers of tubes
(iii) the minimum number of tube passes in each case, if the lengthof a boiler tube is to be less than 5 metres.

Question 3. A fuel gas consists of 75% butane (C4H10), 10% propane (C3H8) and 15% butene (C4H8) by volume.
It is to be fed to the combustion chamber in 10% excess air at 25ºC, where it is completely burnt to carbon dioxide and water. The flue gases produced are to be used to generate 5 bar steam from water at 90ºC.
With the aid of the data at the end of the question, steam tables and the enthalpy table given in the Appendix of lesson HTC - 4 - 2:
(a) Write balanced equations for the combustion of each component of the fuel gas.
(b) Explain the need for excess air.
(c) Determine the actual fuel:air ratio
(i) by volume (ii) by mass.
(d) Calculate:
(i) the net calorific value (CV) per m3 of the fuel/air mix at 25ºC (ii) the net calorific value (CV) per kmol of the fuel/air mix at 25ºC.
(e) Determine the composition of the flue gases by volume (assuming the inlet air is dry):
(i) on a wet basis
(ii) on a dry basis.
(f) Determine the ‘furnace efficiency' if the flue gases leave the boiler at 300ºC.
(g) Give two advantages of preheating the water in this way and one disadvantage.
(h) Give two reasons why the presence of any sulphur in the fuel mix would be undesirable.

Attachment:- Heat Exchangers.rar